STM32F4 OSC Problem

[Scarlet]

Hi everyone

I'm currently using the NUCLEO-F429ZI development board. I'm planning to create a PCB using the STM32F429ZIT6, but I've encountered some questions during the planning stage.

1. According to the datasheet, HSE_OSC is specified to provide 8MHz, and CubeIDE also indicates "Input Freq HSE = 8MHz." However, on the NUCLEO-F429ZI development board, the JFVNY 25.000 oscillator supplies 25MHz. I have verified that the SPI clock indeed derives from multiplying and dividing an 8MHz source, but when I measure the JFVNY 25.000 oscillator on the development board, it does indeed supply 25MHz. Why are these two frequencies inconsistent, or am I misunderstanding something?
2. Referring to the Clock tree and considering, if not using the RTC section, is it possible to omit providing the external LSE_OSE?

[Scarlet]

JFVNY is not mentioned in the document, it is the packaged font on the development board.

[Scarlet]

Yes, X3 is the HSE OSC.

Although I'm not entirely sure about the meaning of 'not provided,' I should be able to design with HSE 8MHz.

Thank you.

[SofLit]

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I should be able to design with HSE 8MHz.

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Yes without Constraints/Issues. And as I said, use PLL to reach your system frequency target.

You can use CubeMx that helps you to find your target frequencies:

[Andrew Neil]

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@Scarlet wrote:

Yes, X3 is the HSE OSC.

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No, it's not the oscillator; it's just the crystal - ie, the resonant element for the oscillator.

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@Scarlet wrote:

Although I'm not entirely sure about the meaning of 'not provided,

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It means it isn't fitted to the board:

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@Scarlet wrote:

I should be able to design with HSE 8MHz.

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With X3 not fitted, you won't be using HSE at all - you'll be bypassing it, and using the 8MHz clock signal from the ST-Link.

[Andrew Neil]

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@Scarlet wrote:

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So I can use STLink to program the microcontroller without providing an external oscillator externally?

However, the MCU requires an HSE of 8MHz for execution. Right?

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The MCU doesn't require HSE at all.

HSE is just one of the many potential clock sources.

The default for the board is to bypass the HSE, and use the 8MHz clock signal provided by the ST-Link

[Scarlet]

My current understanding is that there is no X3 OSC on the development board, and the 8MHz is currently provided by the STLink. Right!?

[SofLit]

Yes. By default X3 is not populated, for external clock source STLink MCO is an alternative.

[Andrew Neil]

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@Scarlet wrote:

My current understanding is that there is no X3 OSC on the development board, and the 8MHz is currently provided by the STLink. Right!?

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Again, X3 is just a crystal - it's not an oscillator:

• The oscillator is within the STM32 - so that is always present;
• The crystal needs to be provided externally - it is not present on the NUCLEO-F429ZI board.

https://www.avrfreaks.net/s/topic/a5C3l000000UYYQEA4/t146620?comment=P-1401006

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@Scarlet wrote:

 the 8MHz is currently provided by the STLink. Right!?

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Yes.

[Scarlet]

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Again, X3 is just a crystal - it's not an oscillator:

Okay, I will research the differences between the two.

Thanks.

[Andrew Neil]

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@Scarlet wrote:

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Okay, I will research the differences between the two.

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Unfortunately, the link in that AVRFreaks post that I referenced seems to be broken.

But here is a description of what's inside a Crystal Oscillator:

https://www.avrfreaks.net/s/topic/a5C3l000000UcE1EAK/t160729

As you can see, an oscillator is a complete active electronic circuit:

It contains a crystal plus  all the active circuitry necessary to make it oscillate and give a nice, clean digital clock signal as output.

The crystal itself is just a passive component:

it requires extra active electronic circuitry to make it oscillate.

In the case of a microcontroller, the oscillator  circuitry is usually inside the microcontroller chip (eg, the HSE within the STM32) - so you just need to provide the crystal externally.

#CrystalVsOscillator #OscillatorVsCrystal